To meet current emission regulations, automotive vehicles must have on-board diagnostic systems to detect the degradation of any component of the emission system, including a catalytic converter.
Catalytic converter monitoring systems are known in which an upstream and a downstream exhaust gas oxygen sensor are compared to give an indication of catalytic degradation. If sufficient degradation occurs, these systems will diagnose the deteriorated performance and indicate a degradation. As a result, a new catalytic converter may be required. When the measured performance of a catalytic converter falls below a predetermined threshold, degradation is indicated. An example of such an approach is disclosed in U.S. Pat. No. 5,357,751.
A better correlation between the measured degradation using the exhaust sensor and actual degradation is obtained when the measured degradation is adjusted based on the airflow through the catalytic converter. This adjustment can be found by numerical optimization and regression techniques for an initial catalytic converter having initial characteristics for catalytic converter monitoring.
The inventors herein have recognized a disadvantage with the above approach. Due to packaging constraints, changing emission regulations, or in-use service, catalytic converters having these initial characteristics are sometimes replaced with catalytic converters having different characteristics. Then, the adjustments that were designed for the initial catalytic converter no longer produce acceptable results in terms of catalytic converter monitoring.
For example, when monitoring a catalytic converter smaller than an initially sized catalytic converter, the performance index, or switch ratio approaches the maximum, or unity, before the emissions have reached the regulated level. Thus, the indication of degradation will falsely indicate that the catalytic converter must be replaced. Alternatively, when monitoring a catalytic converter larger than an initially sized catalytic converter, the performance index, or switch ratio, that represents emissions have reached the regulated level is relatively low and difficult to distinguish from properly functioning catalytic converters. This contrasts with the initial catalytic converter design in which the performance index, or switch ratio just reaches its maximum, or unity, when the catalytic converter just reaches a degradation level to produce emissions at the regulated level.